THE U.S. Congress is set to hear testimony against the British Government's controversial Legacy Bill, which aims to offer amnesty for Troubles killings. 

The House Foreign Affairs Committee was to host a public briefing on truth and accountability yesterday for victims of the conflict. However, due to crisis defense votes, the briefing will now take place from 2pm today, Friday 15 July. 

It comes after weeks of engagement between committee members, the Ancient Order of Hibernians, and Belfast-based human rights organisation Relatives for Justice (RFJ).

The British Government's so-called Legacy and Reconciliation Bill seeks to replace police investigations and due court process with an information "recovery and reconciliation body", which could offer amnesty for Troubles' killers. 

Thursday's briefing, which will be streamed online from 5.30pm, will feature testimony from RFJ CEO Mark Thompson, WAVE Trauma Centre Coordinator Alan McBride and Dr Anna Bryson, Senior Lecturer at QUB's School of Law at the George J. Mitchell Institute for Global Peace.

Speaking ahead of the briefing, Mark Thompson said he will speak to the "illegality of the bill".

He said the British Government's Legacy Bill "violates domestic and international law". 

"It undermines the Good Friday Agreement, the European Convention on Human Rights, and the 1998 Human Rights Act," he said.

"Post-conflict, the Good Friday Agreement built in rights and safeguards to protect rights. It pledges the vindication and protection of human rights for all. This Bill cuts across and violates all of that. 

"In any post-conflict society you should strengthen and safeguard against violations that led to conflict in the first instance. For many families the Good Friday Agreement gave them the agency to pursue justice in an open an transparent way, and the ability to get justice can be achieved to a certain degree. We have institutions like the Police Ombudsman, we have the inquest courts and the civil courts. 

"What we have seen is a rearguard action by the British state, whether it's the MOD or PSNI, fighting families tooth and nail to stop the disclosure of documents and finding out the truth. 

"During the conflict there was a de facto impunity administered through the courts here and the Good Friday Agreement changed that landscape. Now that the families have a more equal playing field it's being taken away from them by the British state, which is concerned with preserving its own narrative."

The Legacy Bill cleared the the House of Commons earlier this month and will be considered by the House of Lords in the coming months.

Mr Thompson said a "small window of opportunity" exists for the U.S. to convince the British Government to abandon the Bill which, he said, is "re-traumatising victims and families".